Reliable and timely detection of acute rejections in renal transplant patients is important to preserve the graft function and to prevent premature graft failure. The current gold standard for the rejection diagnosis is a renal biopsy which is usually performed upon an unexplained decline in the graft function (determined by serum creatinine or clearance). Because of the insensitivity of creatinine determinations and the invasiveness of renal biopsies, non-invasive tests have been suggested to diagnose acute rejection including mass spectrometry (MS) analysis of urine samples.